A Last Meal: Blackheaded ash sawfly munches while it still can

by | May 8, 2019

Countless native insects feed upon our forest trees each year. Typically, their feeding is so minor, it goes undetected and has no long-term effect on tree health. When pest populations do rise, these sporadic and often small outbreaks are unusual and fascinating to nature-lovers.

Last week near Kinston, Jim Slye, N.C. Forest Service Forest Health Specialist, encountered one of our native defoliators, the blackheaded ash sawfly. Called a sawfly, it is actually not a fly, but a primitive wasp. The particular sawfly feeds on ash tree leaves. Typically a pest in ornamental settings, young larvae feed in clusters and cause minor defoliation to entire defoliation of young trees in a couple of weeks. After feeding on the new spring leaves, they drop to the soil where they pupate over the summer, fall, and winter. In the meantime, most ash trees re-leaf and you’d never even know the blackheaded ash sawfly was there.

Blackheaded ash sawfly on green ash. Image: J. Slye, NCFS.

Our ash trees are not foreign to pests feeding on it. In recent years, the invasive emerald ash borer has swept across our state, leaving dead ash trees in its wake. While one perspective may be that ash trees are getting hit left and right by pests, another is that our native blackheaded ash sawfly (which doesn’t kill the tree) is getting in its last meal, so to speak. With increasing detections of emerald ash borer across the state, it is likely just a matter of time before ash trees are eliminated from the N.C. landscape.

Because of its exclusive feeding habits on ash trees, the blackheaded ash sawfly is considered at risk of extinction with the impending loss of ash (Gandhi and Herms 2010). Entire loss of any single tree type often devastates ecosystems and has cascading effects on the countless organisms that rely on them. In fact, invasive species are the second leading cause of organisms being placed on the threatened or endangered species list. The leading cause is human activities.

So, while these small creatures may seem to be harming ash trees, not only is the damage temporary and not long-lasting, but there are much worse offenders out there.

Minor defoliation caused by blackhead ash sawfly. Image: J. Slye, NCFS.